1. Field of the Invention
The present invention relates to an optical scanning unit usable for forming an electrostatic latent image on a surface of an image bearing member such as a photoconductor, and more particularly to an optical scanning unit usable for a multi-beam scanning system having a light source unit including a plurality of light-emitting elements to simultaneously scan using a plurality of light beams for forming images at a high speed, and an image forming apparatus using electrophotography such as a printer, a facsimile machine, a copier, or a multi-functional apparatus having at least two functions of these and employing the optical scanning unit.
2. Description of the Background Art
In image forming apparatuses using electrophotography, light beams emitted from a light source unit are deflected by a rotatable multi-face mirror such as a polygon mirror, and the deflected light beams are focused on an image bearing member such as a photoconductor via optical elements such as a focus lens, a reflecting mirror, or the like to form a desired electrostatic latent image on the image bearing member.
Further, a portion of the light beams deflected by the rotatable multi-face mirror is separated and extracted by the reflecting mirror or the like, and then the separated light beams enter a light detector so that an electrical trigger signal for detecting a scan start signal and a scan end signal are output. In such a configuration, a reflecting mirror for separating and extracting a portion of the light beams may be a glass-based optical member. Further, JP-H08-106039-A discloses a technique using a metal block having received specular working, which corresponds to a reflecting mirror made from a glass-based member.
As more and more multi-color image forming apparatuses have come to be equipped with high speed printing capabilities, and more and more image forming apparatuses have come to be used as on-demand printing systems, users have been demanding more precise imaging from the image printing system. In light of such trend, JP-2009-47924-A discloses an image forming apparatus using a two-dimensional surface-emission laser array such as a vertical cavity surface emitting laser (VCSEL) configured with a plurality of light-emitting sources, in which a plurality of light beams is used to scan an image bearing member simultaneously to form a plurality of lines of an electrostatic latent image simultaneously, which may be referred to as a multi-beam scanning system. Such multi-beam scanning system disclosed in JP-2009-47924-A can be devised as a prominent technology for an optical scanning unit that can form more precise images at a high speed.
In general, image forming apparatuses may employ various types of light sources such as the above-mentioned two-dimensional surface-emission laser array having a plurality of light-emitting elements or a conventional single light-emitting source such as an edge-emission laser. However, regardless of type of light source, the intensity of the light flux emitted from the light source may fluctuate due to temperature changes or simply aging, and thereby fluctuation may occur in the density of images output by the image forming apparatus.
To reduce the fluctuation of light intensity of light flux, in an optical scanning unit using a conventional edge-emission laser, a light beam emitted rearward of the light source may be monitored to adjust the light intensity of light source automatically, which is known as auto power control (APC control). However, in the multi-beam scanning system, the above-mentioned two-dimensional surface-emission laser array having a plurality of light-emitting elements does not emit light beams rearward of the light-emitting elements due to its inherent configuration. Therefore, a portion of the light flux emitted as light flux for forming a latent image may be separated and extracted at a light path, and such extracted light is further reflected and guided to a light detector such as a photodiode for use as a monitor. Then, the APC control may be conducted using light intensity of the monitor-use light flux received by the light detector.
Such APC control can be conducted by using the metal block having received specular working (as disclosed in JP-H08-106039-A) or by using a conventional glass-based optical member to extract a trigger signal. However, the metal block having received specular working needs a specular working process, which increases manufacturing costs. Similarly, in the case of using a conventional glass-based optical member as reflecting mirror, a plurality of reflecting mirrors and a prism may be required to extract and reflect a portion of the light flux, emitted from a light source, to a light detector disposed at a given position, which also increases manufacturing costs as well as the size of apparatus. As market demand has shifted to lower-cost machines, such configuration may not be cost-effective.
In JP-2009-47924-A, a light-flux splitter is disposed, in which the light-flux splitter has an aperture having a diameter smaller than the light flux coming from a light source such as two-dimensional surface-emission laser array. The light-flux splitter has a reflecting face having higher luminescence and faces the light source, while the reflecting face is angled with respect to an optical axis to a given angle such as 45 degrees. In such a configuration, the light flux passing through the aperture of light-flux splitter strikes a rotatable multi-face mirror such as a polygon mirror to form an electrostatic latent image on an image bearing member.
Further, any light flux not passing through the aperture of light-flux splitter (hereinafter surrounding light beam) is reflected by the reflecting face of light-flux splitter and further reflected by a convergent lens and a reflecting mirror toward a circuit board having the light source. A light detector disposed on the circuit board detects the light intensity of the surrounding light beam, and based on the detected light intensity of the surrounding light beam, APC control can be implemented.
In such a configuration detecting the light intensity of light beam from a light source, the APC control can be implemented with a relatively lower cost. However, as described above, users seek more precise images, and to produce those more precise images, the APC control needs to be conducted more precisely or strictly. Accordingly, the surrounding light beams extracted from the light-flux splitter needs to be detected more precisely to conduct the APC control more precisely.
When a two-dimensional surface-emission laser array such as a VCSEL is used as a light source, a portion of a plurality of light beams emitting from a light source having a plurality of light-emitting elements strikes a light detector. A beam spot diameter of the light beams coming from all light sources needs to be a desired size when the light beams enter the light detector. Such desired beam spot diameter may be determined in view of an assembly configuration and assembly precision of the light-flux splitter.
Further, to conduct the APC control more precisely, it is preferable that the light beams, reflected by the light-flux splitter and entering the light detector, are not mixed with other disturbance light as much as possible. Because the light-flux splitter angled with respect to the optical axis of the light flux coming from the light source, a portion of the light beam passing through the aperture of light-flux splitter may be reflected at a thicker part of light-flux splitter and may interfere with the monitor-use reflected light beams guided to the light detector.
Accordingly, the light-flux splitter is preferably configured as thin as possible to reduce disturbance light reflected at the thicker part of light-flux splitter. However, the thinner the light-flux splitter, the more warping occurs for the light-flux splitter during manufacture. If the light-flux splitter is used without correcting such warping, light beams may not be reflected at the light-flux splitter with a desired reflection angle, and APC control precision may deteriorate.